[fr8train]

I used to have this same problem. Touch ups not sanding out no matter how tight I feathered them. Then I realized it was actually worse the tighter I put it on. Thats when it dawned on me to try leaving the edges a little heavier.. and voila my touch ups sand out perfectly now. It seems contrary to logic but it works

There's gotta be something there to sand. Same problem as pulling a coat too tight.

 

[fr8train]

Stopper, if your mud is drying up that hard on the chain, you need to let it sit in a bucket of water when you aren't running. Keeps all that mud wet. We tape all flats, take a break, the taper head is soaking at this time. When we start angles, the head gets scrubbed. A crummy head while running angles will cause you grief. If it gets crusty while running angles we'll scrub it again.

 

[fr8train]

Different batches of mud have different amounts of water, and therefore different densities. Could be you are touching up with a heavier/denser mix, than what you used for skimming. try touching up with the same mud. I seem to remember a discussion on this a while back. However, it was probably a thread that went off topic, so good luck finding it!